The blast furnace is a tall shaft-type furnace with a vertical stack superimposed over a crucible-like hearth. Iron-bearing materials (iron ore, sinter, pellets, mill scale, steelmaking slag, scrap, etc.), coke and flux (limestone and dolomite) are charged into the top of the shaft. A blast of heated air and also, in most instances, a gaseous, liquid or powdered fuel are introduced through openings at the bottom of the shaft just above the hearth crucible. The heated air burns the injected fuel and much of the coke charged in from the top to produce the heat required by the process and to provide reducing gas that removes oxygen from the ore. The reduced iron melts and runs down to the bottom of the hearth. The flux combines with the impurities in the ore to produce a slag which also melts and accumulates on top of the liquid iron in the hearth. The iron and slag are drained out of the furnace through tapholes.
The top pressure that is controlled by the top gas handling equipment can be as high as (40-50 psig) for very large furnaces, and the blast air has been enriched with oxygen as high as 40% total oxygen in the blast. Pressure at the inlet of the tuyeres depends on the controlled top pressure and the quality of the raw materials, but can be as high as 60 psig for a very large blast furnace. Oxygen enrichment reduces the amount of air needed per tonne of iron and therefore, the resulting quantities of BF Top Gas are reduced.
Ion transport membranes (ITMs) consist of ionic and mixed-conducting ceramic oxides that conduct oxygen ions at elevated temperatures of 1475-1650 F. Air is compressed to about 230 psia, heated to 1650 F, and fed to ITM. Hot oxygen permeates through the membranes. The permeate pressure has to be kept low to provide oxygen partial pressure driving force across the membrane. Typically, 50% to 80% oxygen recovery seems possible.